Ore grinding control



1968 B. s. CROCKER ETAL 3,417,927

ORE GRINDING CONTROL Filed June 30, 1965 Fine Ore ll I Feed i l Rod Mill l To Second I |3 Pebble Mill Section l l6 l5 I 2 Cyclone To Sl mes 1/1 I Floro'rlon Pebble Feed l r' l i 2? l i Pebble Mill 5" L I 22 l i 2| l g' 20 l I 2 Cyc|one I TO Sopds l Florclllon I l Q l l l l l J -2e 24 mveu'rons Bunting S.Crocker T.G Fulmor F.l Holderreed Wllllom Lucy av J.F. Muhoney 2& n dmnl$, M6046", 7ybr 8- 14M; ATTORNEYS United States Patent 3,417,927 ORE GRINDING CONTROL Bunting S. Crocker, Toronto, Ontario, Canada, and Theodore G. Fulrnor, Francis L. Holderreed, William Lucy, and John F. Mahoney, Anaconda, Mont., assignors to The Anaconda Company, New York, N.Y., a corporation of Montana Filed June 30, 1965, Ser. No. 468,505 5 Claims. (Cl. 24124) ABSTRACT OF THE DISCLOSURE An ore grinding control system is shown in which radiation gauge means are used to measure the level and density of the circulating load of a grinding mill. The output signal of the density gauge is transmitted to controller means to regulate either the ore feed input to the grinding mill or the ore feed to a preceding grinding stage. The system additionally provides for power sensor means to measure the power consumption of the grinding mill and controller means responsive to the measured power for regulating the ore feed input to the grinding mill.

This invention relates to ore grinding and, more particularly to a method and system for controlling an ore grinding process in which a circulating load is passed through an autogenous grinding mill.

Modern ore processing, such as a copper concentration operation, involves a complex of processing steps, including stages of crushing, grinding, flotation, and thickening. A large installation may have an ore grinding capacity of several thousand tons per hour and employ an elaborate arrangement of rod mills, pebble mills, cyclone separators and pumps to reduce the ore coming from the crushers to a size and to form and circulate a pump of consistency suitable for submission to the flotation processes, where the actual concentration or separation of the copper content from the ore takes place. In order to maintain continuous ore processing with a maximum of performance efiiciency, such an ore grinding operation must be carefully controlled.

In a modern ore grinding operation fine ore from the crushers is fed into a rod mill which continuously grinds the ore and discharges its output through a cyclone classifier into an autogenous pebble mill..In addition to this fine ore input from the rod mill the pebble will also is fed pebbles, or rock size particles of ore, by a variable pebble feed input. As an autogenous grinding mill, the pebble mill employs only this ore material-pebbles and fire ore-in a slurry with water and lime, to effect its grinding action. Thus the ore materials grind against themselves as the pebble mill rotates, without any grinding action by iron balls or rods as in ball or rod mills.

The pebble mill grind is discharged into a second cyclone separator which separates out the coarser ore material and diverts it back into the input to the pebble mill to form a continuous circulating load through the pebble mill.

For maximum performance and efliciency in such an ore grinding section, it is necessary to observe the level and density (i.e., the mass flow) of the circulating load carefully and to regulate the factors which afiect the density and level of the circulating load. In the past such control has been difficult and relatively ineffective. It has been impossible to observe the changes in ore grind density as, for example, when the characteristic of the fine ore or pebbles introduced into the pebble mill changed from hard to soft, with sufiicient accuracy or speed to permit the effective control of the factors effecting grinding operation.

We have discovered a method and a system for maintaining effective control of an ore grinding process in which a circulating load is passed through an autogenous grinding mill. Our system employs a radiation type gauging unit for measuring the density and level of the circulating load and means for regulating fine ore feed input and pebble feed input to the autogenous grinding mill responsive to the value of density and level measured by a gauging unit. Our system in a preferred embodiment further makes use of an electric power sensing means to measure the electric power consumed by the grinding mill and means for regulating the pebble feed input to the grinding mill responsive to the value of power measured by the power sensing means.

These and further objects and advantages of our invention will be more readily understood when the following description is read in connection with the accompanying drawing, in which:

The single figure is a schematic diagram of a subunit of a copper ore grinding operation and a preferred embodiment of our control system.

A typical portion of a copper ore grinding operation is shown in the drawing. Fine ore from ore crushers is fed by means of a variable fine ore feed input 11 into a rod mill 12. Such a rod mill has a capacity of 6,000 tons per day, grinding one-inch ore. Lime and water are also fed into the rod mill 12, but these inputs have not been shown in the single figure. The output of the rod mill 12 is divided into two streams at 13, each of which goes to a cyclone associated with a pebble mill. For the sake of simplicity only one of these pebble mill sections has been illustrated in the single figure. The operation of the other pebble mill section duplicates the operation of the pebble mill section shown and to be described.

The rod mill 12 discharges into a cyclone separator 14 which separates the ore material into finer ore material which is expelled from the cyclone overflow 15, proceeding to a slimes flotation process, and coarser ore material is discharged from the cyclone spigot 16 into an autogenous pebble mill 17. In addition to the material from the rod mill cyclone 14, the pebble mill 17 is also fed pebbles, rock size particles of ore, from a variable pebble feed input 18. Although not shown in the drawing, lime and water are also added to the pebble mill.

The pebble mill grind is discharged into a second cyclone 19 which dixides the grind material into a finer grade which flows from the cyclone overflow 20 to a sands flotation process and a coarser grade which is emitted at the cyclone spigot 21 and is pumped back to the input of the pebble mill 17 to form a circulating ore load loop 22. v

In order to measure the level and density of the circulating load, a radiation type gauge 23 is mounted externally on the circulating ore load loop 22. This gauge 23 makes use of a source of radiation, such as a radioisotope, and a radiation detector mounted on opposite sides of the load loop 22. As the density and level of the material passing between the gauge source and detector change, the amount of radiation sensed by the detector changes and the electrical output signal from the gauge detector varies. In this manner, the mass fiow (the combined value of density and level, the latter being a measure of volume) of the circulating ore load is converted by the radiation gauging unit 23 into an electric signal.

The radiation gauging unit 23 is connected to a control device 24 comprising electrical circuit means for converting the electric output signal from the gauge unit 23 into a form suitable for controlling either the fine or feed input 11 to the rod mill 12 or the pebble feed input 18 to the pebble mill 17. As noted before, these ore inputs are variable, and their rate of ore transfer may be regulated by controlling the electrical voltage energizing their driving means. A number of methods for converting an electric analog output signal from a gauging means to a form suitable for controlling the feed rate of an ore input are available and are Well known to those skilled in electrical or electronic art. In calibrating the control system a standard optimum value of level and density for the desired mass flow is selected and the value of the electric signal from the gauging unit 23 corresponding thereto is determined. The control device 24 is so designed that if it is connected to the fine ore feed input 11 as shown by the broken line 25, the fine ore feed rate to the rod mill 11 is decreased in response to a measurement by the gauging unit 23 of a mass flow greater than the optimum value. Thus, if the circulating ore characteristic changes to hard and the circulating load increases the rod mill discharge from the cyclone spigot 16 into the pebble mill 17 decreases, tending to restore the mass flow of the circulating load to the optimum value. Similarly, when the mass flow of the circulating load decreases the fine ore feed rate to the rod mill 12 is increased thereby causing more ore to be input to the pebble mill 17.

If the control device 24 is connected in its second mode of operation, indicated in the single figure by the broken line 26, the ore feed rate of the pebble feed input 18 to the pebble mill 17 is similarly controlled to maintain circulating load level and density at the optimum value.

An additional feature of a preferred embodiment of our control system further provides for an electrical power sensing device 27 connected to the pebble mill 17 to measure the electric power consumed in driving the pebble mill. A standard thermal converter power sensor is used for this purpose. The sensor 27 produces a signal proportional to the power consumed by the mill motor. The sensor 27 is connected to a second controller 28 which, like device 24, converts the signal from the sensor to a level suitable for regulating the pebble feed input 18. Since the pebble load in the mill is determinative of the power and grinding ability of the pebble mill 17, an optimum power load can be maintained by regulating the pebble feed to the mill. The controller 28 has added safety circuitry to actuate an alarm when the pebble mill 17 is nearing overload and to stop the pebble feed input 18 completely just short of overload. Provision is also made to restart the pebble feed automatically when the load falls below a preset limit. Normally the grinding is continuous without shutdown and these last provisions are only made in case of a sudden process upset.

Thus, it may be seen that by continuously measuring the mass flow of the ore in the circulating load passing through the pebble mill and continuously regulating at least one of the variable ore feeds to the pebble mill in response to instantaneous variations of the mass flow of the circulating load we are able to maintain precise and eifective control over the pebble mill grinding circuit, not obtainable previously. Moreover, by simultaneously measuring the electric power consumption 9f 1. pebble mill and continuously regulating the pebble feed input to the pebble mill in response to the instantaneous variations in the power consumed by the pebble mill, an additional means of control is provided. Not only may the immediate ore inputs to the pebble mill be regulated in our system, but as described above, the ore feed input to a preceding grinding stage, such as the rod mill, is controllable in response to variations in circulating load density. A great advantage of our system lies in the ability of the operator to determine which of the ore feed inputs to the pebble mill is to be regulated in by the density gauge and controller. This option in selecting either the immediate pebble feed input to the pebble mill or the fine ore feed input to the rod mill for regulation gives the operator great flexibility in exercising the precise degree and nature of control over the grinding process. Our system therefore controls an ore grinding process with a precision, speed, flexibility and efliciency not available previously.

It will be understood that various changes in the details, materials, steps and arrangement of parts which have been herein described and illustrated in order to explain the nature of our invention may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

We claim:

1. A system for controlling an ore grinding process in which a circulating load is passed through an autogenous grinding mill having a first variable ore feed input and a second ore feed input from a preceding grinding stage, said preceding stage having a variable ore feed input, the system comprising a radiation type gauge for measuring the mass flow of the circulating load and controller means responsive to the valve of mass fiow measured by the gauge for optionally regulating the first variable ore feed input and for optionally regulating the variable ore feed input of the preceding grinding stage.

2. A system for controlling an ore grinding process in which a circulating load is passed through an autogenous grinding mill having a first variable ore feed input and a second ore feed input from a preceding grinding stage, said stage having a variable ore feed input, the system comprising a radiation type gauge for measuring the mass flow of the circulating load, a power sensor for measuring the electric power consumed by the grinding mill, first controller means responsive to the value of mass flow measured by the gauge for regulating the variable ore feed input of the preceding grinding stage, and second controller means responsive to the value of power measured by the sensor for regulating the first variable ore feed input.

3. The method for controlling an ore grinding process in which a circulating load is passed through an autogenous grinding mill having a first variable ore feed input and a second ore feed input from a preceding grinding stage, said stage having a variable ore feed input, comprising the steps of continuously measuring the mass flow of the circulating load, continuously measuring the electric power consumed by the grinding mill, continuously regulating the variable ore feed input of the preceding grinding stage in response to instantaneous variation in the mass flow measured and continuously regulating the first variable ore feed input in response to instantaneous variations in the power measured.

4. A system for controlling an ore grinding process in,

which a circulating load is passed through an autogenous grinding mill having a variable ore feed input, the system comprising a radiation type gauge for measuring the density of the circulating load, a power sensor for measuring the power consumed by the grinding mill, and controller means responsive to the value of density measured by the gauge and to the value of power measured by the sensor for regulating the variable ore feed input.

5. The method for controlling an ore grinding process in which a circulating load is passed through an autogenous grinding mill having a variable ore feed input comprising the steps of continuously measuring the density of the circulating load by means of a radiation type gauge, continuously measuring the power consumed by the grinding mill and continuously regulating the variable ore feed input in response to variations in the measured density and power.

References Cited UNITED STATES PATENTS 2,965,316 12/1960 Henderson et al 241--34 3,352,499 11/1967 Campbell 24121 2,499,347 3/ 1950 Adams 241-34 6 6/ 1963 Fahlstrom et al. 241-34 4/1966 Franz 241-34 FOREIGN PATENTS 7/1963 Canada.

OTHER REFERENCES 10 ANDREW R. J UHASZ, Primary Examiner.

US. C1.X.R. 

